1. Field of the Invention
The embodiments described herein relate generally to a collimator for use in X-ray imaging systems and, more particularly, to a secondary collimator for use with an X-ray diffraction imaging (XDI) system.
2. Description of Related Art
At least some known security detection devices utilize X-ray imaging for screening luggage. For example, XDI systems provide an improved discrimination of materials, as compared to that provided by more conventional X-ray baggage scanners, by measuring d-spacings between lattice planes of micro-crystals in materials. A “d-spacing” is a spacing between adjacent layer planes in a crystal.
At least one such XDI system that uses an inverse fan-beam geometry (a large source and a small detector), such as a multiple inverse fan beam (MIFB) topology, and a multi-focus X-ray source (MFXS) has been proposed. To allow examination of objects having a width of up to about 1 meter (m), a relatively large number of detector elements are required. At least one known XDI system includes a secondary collimator defined by an array of slits in a series of high Z (tungsten alloy) baffles. A “high Z” material is a material having a high atomic number, such as, for example, tungsten (Z=74), platinum (Z=78), gold (Z=79), lead (Z=82), and/or uranium (Z=92). However, such a secondary collimator does not permit the number of detector elements to be increased because the baffles cannot be fabricated to include a high number of slits without adversely affecting the operability of the secondary collimator. Moreover, such known secondary collimators are difficult and expensive to manufacture because the collimators are fabricated from tungsten alloy.
Another known collimator for use with X-ray investigation systems is a Soller slit collimator. At least some known Soller slit collimators commonly include a stack of continuous plates that are regularly spaced with respect to each other. If a plate separation is P and a length of the Soller slit collimator in a propagation direction is L, a maximum angular divergence, Δθof a beam emerging from the Soller slit collimator is equal to about P/L, a parameter that is also known as the aspect ratio.
The MIFB topology of an XDI system requires a fixed angle secondary collimator (FASC) that is, in principle, a stack of Soller slit collimators having a relatively high aspect ratio. An MIFB FASC can include up to 25 plates stacked parallel to each other with a pitch of about 1.25 mm, which yields 24 channels each with a Δθof about 1 milliradian (mrad). Such an FASC covers an extent of about 2500 millimeters (mm) in a Y dimension. However, if such an FASC is built using known techniques, the FASC would require plates having a 2.5 meter (m) width (Y) and a 0.75 m length (X), which are separated from each other by about 1.25 mm. The spacing and planarity of such plates must be held constant to a tolerance of 0.1 mm, however there are no known methods of producing such a large, low divergence collimator.
At least some other collimators having a Y dimension and a pitch as described above have a much smaller length than is desired for an FASC. For example, at least some computed tomography (CT) machines have anti-scatter grids with a length (in a Z-direction) of about 10 centimeters (cm), a pitch of about 1.25 mm, and a septa thickness of about 0.5 mm; however a height in an X-direction of travel of the X-rays for such a grid is only about 20 mm. As used herein, the term “septa” refers to walls or partitions that separate spaces, slits, cavities, slots, chambers, and/or other openings. Further, because of the lower height of the CT grid as compared to the about 0.75 m height of the FASC, fabrication techniques for forming the CT grid, such as maintaining slit spacing by holding plates at their edges using thin wires, cannot be applied to forming an FASC. Moreover, because of the size difference, the CT grid and the FASC each have different structure, design, and/or material choice considerations.
As such, it is desirable to provide a method for manufacturing an FASC that is mechanically precise and large enough to be used with an XDI system. Further, it is desirable to manufacture an FASC from a certain number of identical building blocks.